The invention relates to systems and methods for communications of digitally modulated signals, and more particularly to systems and methods utilizing multiple bands including,. e.g., parts of an amplitude-modulation (AM) frequency band, to communicate digitally modulated signals.
The explosive growth of digital communications technology has resulted in an ever-increasing demand for bandwidth for communicating digital audio information, video information and/or data.
For example, to efficiently utilize bandwidth to communicate digital audio information, a perceptual audio coding (PAC) technique has been developed. For details on the PAC technique, one may refer to U.S. Pat. No. 5,285,498 issued Feb. 8, 1994 to Johnston; and U.S. Pat. No. 5,040,217 issued Aug. 13, 1991 to Brandenburg et al., both of which are hereby incorporated by reference. In accordance with such a PAC technique, each of a succession of time domain blocks of an audio signal representing audio information is coded in the frequency domain. Specifically, the frequency domain representation of each block is divided into coder bands, each of which is individually coded, based on psycho-acoustic criteria, in such a way that the audio information is significantly compressed, thereby requiring a smaller number of bits to represent the audio information than would be the case if the audio information were represented in a more simplistic digital format, such as the PCM format.
Recently, the industry turned its focus to the idea of utilizing the preexisting analog AM frequency band more efficiently to accommodate digital communications as well. However, it is required that any adjustment to the AM band to provide the additional capacity for digital communications does not significantly affect the analog AM signals currently generated by radio stations on the same band for AM radio broadcast. In the United States, adjacent geographic areas covered by AM radio broadcast are assigned different AM carrier frequencies, which are at least 20 kHz apart. Specifically, when they are exactly 20 kHz apart, the AM carrier assigned to the adjacent area is referred to as a xe2x80x9csecond adjacent carrier.xe2x80x9d Similarly, when they are 10 kHz apart, the AM carrier assigned to the adjacent area is referred to as a xe2x80x9cfirst adjacent carrier.xe2x80x9d
An in-band on channel AM (IBOC-AM) (also known as xe2x80x9chybrid IBOC-AMxe2x80x9d) scheme utilizing bandwidth of the AM band to communicate digital audio information has been proposed. In accordance with the proposed scheme, digitally modulated signals representing the audio information populate, e.g., a 30 kHz digital band centered at an analog host AM carrier. The power levels of the spectrums of the digitally modulated signals are allowed to be equally high across a 10 kHz subband in the digital band on each end thereof.
However, in implementation, it is likely that two such IBOC-AM schemes would be respectively employed in two adjacent areas, to which the host AM carriers assigned are 20 kHz apart. In that case, the 30 kHz digital bands for digital communications centered at the respective host AM carriers overlap each other by 10 kHz, thereby causing undesirable xe2x80x9cadjacent channel interferencexe2x80x9d to each area. In particular, such interference is referred to as xe2x80x9csecond adjacent channel interference,xe2x80x9d as the dominant interfering carrier in this instance consists of a second adjacent carrier. For example, the second adjacent channel interference degrades the digital communications in each of the adjacent areas, especially in the parts of the areas which are close to their common border.
Accordingly, there exists a need for a technique, e.g., based on the PAC technique, for effectively utilizing the AM band for digital communications and treating adjacent channel interference in adjacent areas where IBOC-AM schemes are employed.
In accordance with the invention, in communicating a signal over multiple frequency bands including, e.g., in parts of the AM frequency band, multistream coding is implemented, whereby multiple digital representations each containing information descriptive of the signal are generated. The information contained in at least one of the representations is different than that contained in every other representation. In an illustrative embodiment, at least one of the representations (referred to as a xe2x80x9ccore representationxe2x80x9d) contains core information, and the remaining non-core representations (referred to as xe2x80x9cenhancement representationsxe2x80x9d) contain enhancement information. The core information is more generally descriptive of the signal than the enhancement information. Each representation is transmitted through the frequency band assigned thereto, thereby realizing multistream transmission.
The aforementioned signal may be recovered using all of the digital representations or a subset thereof if some of the frequency bands are severely affected by, e.g., the first or second adjacent channel interference caused by the first or second adjacent channel carrier described above in the case of the IBOC-AM system. The quality of the recovered signal varies with the actual representations used. The signal recovered using only the core representation has the minimal acceptable digital quality. The signal recovered using the enhancement representations, in addition to the core representation, has relatively high quality. In the latter case, the more enhancement representations are used, the higher the quality. However, without the core representation, no signal of acceptable digital quality can be recovered.
Thus, in accordance with an aspect of the invention, the frequency band which is the least susceptible to the interference is assigned to the core representation for transmission to improve the chance of recovery of a signal having at least acceptable digital quality. Advantageously, for example, relative to the prior art IBOC-AM system, an IBOC-AM system implementing the multistream transmission scheme described above affords increased robustness against adverse channel conditions, and more graceful degradation of digital communications when such conditions occur.